86 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
with these mesodermal folds of Amphzoxus. Не has observed that in both asci- 
dians and Amphzoxus these cells are more rounded than other cells of the gastrula. 
Like Klaatsch, Morgan and Hazen find that around and within the ventral lip of 
the blastopore, during the early gastrula stages, there are frequently found small 
rounded cells which contain little yolk. They affirm that the form of these cells 
is not the result of cell division, as Samassa had assumed, but that they preserve 
their rounded form even in the resting stage. 
Samassa (1898), however, says that in Amphzoxus the origin of the mesoderm 
has no relation to the blastopore. Тһе fact that the mesoderm has it growth 
zone at the caudal end of the embryo, in the vicinity of the blastopore, is, he says, 
a condition which it shares in common with all other organs of the embryo. In 
the face of the positive evidence adduced by Lwoff, Klaatsch, Morgan and Hazen 
this negative conclusion of Samassa's seem to me to lose much of its weight. 
It seems probable from these accounts that mesoderm cells are present in 
the ventral lip of the early gastrula of Amphzoxus just as in the ascidians, and 
that they give rise to the longitudinal mesodermal folds of later stages; it remains 
to beseen whether these mesoderm cells may not be traced back to a still earlier 
stage, comparable with the crescent in the ascidian egg (cf. text figs. XXVII- 
XXXII). : 
The origin of the mesoderm in amphibians is a much more difficult and com- 
plicated question and one into which I cannot enter fully here. It is generally 
believed, however, that in the frog’s egg the cells which are to form the mesoderm 
are present when the dorsal lip first appears, and even prior to that time. They 
are the deeper layer of cells of the blastoporic ring and, therefore, surround the 
egg below the equator. Whether at their first appearance they surround the entire 
blastopore is not plain, but in later stages this is said to be the case. According to 
this view the notochord is a mesodermal structure differentiated out of the con- 
tinuous ring of mesoderm surrounding the blastopore. There is here resemblance 
to the chorda-mesenchyme ring which is present in the ascidians and probably also 
in Amphioxus, but in the amphibians this ring appears to give rise at once to a 
sheet of: mesoderm and not to mesodemal bands such as are found in Amphioxus 
and ascidians (text figs. XXXIII-XXXVIII). 
On the whole it is probable that there is fundamental agreement between 
Amphioxus and ascidians in the place and manner of mesoderm formation, апа. 
though the amphibians differ in some important respects from the other two classes 
it is possible to interpret their method of mesoderm formation in the same general 
terms. 
Referring to Rabl’s (1892) “Theorie des Mesoderms," Samassa (1898), and 
Garbowski (1898) maintain that there is no “ peristomal" mesoderm in Amphioxus, 
but that all the mesoderm is “ gastral.” If the view here taken is correct, all the 
mesoderm of this animal is at first peristomal while the gastral mesoderm is later 
derived from this. This is exactly the conclusion which has been reached by Davidoff 
(1891), and Castle (1896), with regard to the ascidian, a conclusion which I can 
